The present invention relates to a method of, and radio terminal for, detecting the presence of a 2-FSK signal. The radio terminal may typically be a telemetry module or part thereof for use in remote monitoring applications such as automatic water metering.
Telemetry modules are installed in equipment which may be continuously in use for many years without being serviced. In the case of battery powered telemetry modules it is desirable for them to operate for up to 10 years between battery replacements. To be able to achieve such long service lives the telemetry modules operate in accordance with a protocol facilitating current saving whilst giving an adequate response time. Protocols achieving these objectives are well known in various technical fields such as digital paging in which the CCIR Radiopaging Code No. 1, alternatively known as POCSAG, has been in use for nearly 20 years. The general approach followed is that the radio unit xe2x80x9csleepsxe2x80x9d for long periods of time but wakes up periodically to check if there are any data signals being transmitted on its channel. The wake-up period may be preset independently of whether or not signals are present. In a refinement of this type of battery economy protocol, when the radio unit has been woken-up, it checks for the presence of data before energising the entire receiver and if none is detected within a period of time which is shorter than the preset period, it powers down prematurely. Consequently battery life may be extended or a smaller sized battery used with the same electronic device.
If the radio unit is unreliable in detecting data then firstly there is a probability of a false alarm (P (false alarm) or P(fa) for short) which is defined as the probability that a signal is xe2x80x9cdetectedxe2x80x9d by a data presence detector, even when only noise is present, and secondly there is a probability of false dismissal (P(false dismissal) or P(fd) in short) which is defined as the probability that the data presence detector rejects a good signal and takes it for noise. P(fd) is a more critical parameter because every single false dismissal of data will cause a complete loss of a packet. Typical system requirements are: P(fa)xe2x89xa61% and P(fd)xe2x89xa60.1%
A block schematic diagram of a typical data presence indicator is shown in FIG. 1 of the accompanying drawings. In FIG. 1 a signal received by an antenna 10 is frequency down converted in an r.f. front end stage 12. The stage 12 comprises a quadrature frequency down conversion stage which provides a real output I(t) and an imaginary output Q(t) at a zero IF or low IF. These outputs are applied to respective squaring circuits 14, 16 which produce outputs I(t)2 and Q(t)2, respectively. These signals are summed in a stage 18 and the square root of the sum is derived in a square root stage 20. The output       P    ⁡          (      t      )        ⁢      (          =                                                  I              ⁡                              (                t                )                                      2                    +                                    Q              ⁡                              (                t                )                                      2                                )  
is applied to a running sum stage 22. The running sum from the stage 22 is compared with a fixed threshold value generated by a stage 24 in a comparator 26, an output 28 from which comprises a data presence indication.
The main purpose of the illustrated indicator circuit is to distinguish a signal from noise. A drawback to this known circuit is that any factors present which reduce the distinctiveness of the signal relative to noise will slow down the detection mechanism rendering it less economical from a power consumption point of view.
An object of the present invention is to optimise the detection of the data presence for the shortest possible time that guarantees the minimum reliability required whilst minimising the energy requirements.
According to one aspect of the present invention there is provided a method of detecting the presence of a 2-FSK signal, the method comprising receiving a 2-FSK signal, quadrature frequency down-converting the received signal to produce quadrature related outputs, oversampling the quadrature related outputs to produce digital samples, differentially decoding the digital samples to produce real and imaginary components, integrating the imaginary components and comparing the integrated value with a fixed threshold value and determining a signal to be present if the threshold is exceeded.
According to another aspect of the present invention there is provided a radio terminal comprising means for receiving a 2-FSK signal, a quadrature frequency down converting means having an input coupled to the signal receiving means and outputs for quadrature related signals, decoding means for decoding the quadrature related signals to produce real and imaginary components, means for integrating the imaginary components, comparison means having a first input coupled to receive an output from the integrating means, a second input coupled to a threshold generating circuit and an output for providing a signal presence indication when the output from the integrating means exceeds a value produced by the threshold generating circuit.
The present invention is based on enhancing the signal relative to noise by reducing the addition of pure noise to a data presence indicator thereby increasing the speed of determining the presence of a signal without reducing the level of reliability. The speed increase may be up to 9 times faster. Therefore if no signal is present the terminal may be deactivated sooner, thus enhancing the battery life.
In the case of 2-FSK modulated transmissions the constellation of decoded signals lies totally on the imaginary branch. Thus integrating data registered on the real branch, which data is similar to pure noise, will not be beneficial.